Flow regulator device for an analytical circuit and its use in chromatography

ABSTRACT

A flow regulator device for an analytical circuit, characterised by comprising: a fluid restriction (R) of defined characteristics related to the field of application of the analytical circuit, a direct-pressure regulator ( 2 ) positioned upstream of said restriction (R), a back pressure regulator ( 4 ) positioned downstream of said restriction (R).

This invention relates to a flow regulator device for an analyticalcircuit and its use in chromatography.

In the technical chromatography sector it is often required to controlthe flow through the analytical circuit, both in the sense ofmaintaining it constant and in the sense of varying it in the requiredmanner for analytical purposes.

The requirement to maintain the flow constant during chromatographicanalysis is related to the fact that the hydraulic column loads varywith varying temperature during analysis, the requirement to vary theflow in a required manner being instead related to the need to subjectthe sample to different analysis methods.

To control the flow through a column of an analytical circuit, apressure regulator is currently used in series with the column. For thispurpose a control program is provided which varies the gas pressure insuch a manner as to compensate the column load variations, due forexample to temperature variations, which in their turn are related forexample to the analysis program.

A drawback of this system is the fact that it is strictly related to thecolumn characteristics and does not provide a general solution to theproblem, given the differences between one column and another.

An actual flow regulator in the form of a mass flow regulator is alsoknown for connection upstream of the column. It is based on the coolingeffect provided by a gas stream which strikes a hot filament. As theextent of the cooling effect depends on the gas flow, the flow can becontrolled by controlling the temperature of said filament.

A drawback of this flow regulator is the fact that besides beingsensitive to the gas flow it is also sensitive to the gascharacteristics; the result is that a variation in the filamenttemperature does not only signify a variation in the flow rate of thegas to be analysed, but can also depend on a variation in the gascomposition; consequently a regulator of this type cannot be used in ananalytical circuit, the function of which is precisely to determine thegas composition.

An object of the invention is to provide a flow regulator device whichis free of the aforesaid drawbacks.

Another object of the invention is to provide a flow regulator devicewhich can be effectively used in circuits in which variations in thecomposition of the gas to be analysed may occur.

These and further objects which will be apparent from the ensuingdescription are attained, according to the invention, by a flowregulator device for an analytical circuit, as described in claim 1.

A preferred embodiment of the invention is described in detailhereinafter, together with some of its particular advantageousapplications in the chromatographic field, with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic view of a device according to the invention.

FIG. 2 shows schematically the use of a device of the invention to forma standard throttling circuit,

FIG. 3 shows schematically the use of devices of the invention toelectronically control the split of a gas chromatograph, and

FIG. 4 shows schematically the use of the device of the invention in amulti-circuit analytical circuit to implement chromatograph switching.

As can be seen from the figures, the flow regulator device of theinvention comprises a fluid restriction R of constant characteristics,which can consist for example of a piece of tube of length L anddiameter D, presenting at its inlet end a pressure P_(i) and at itsoutlet end a pressure P_(u)≦P_(i).

As is known from the Poiseuille equation, the flow F of a gas passingwith very low linear velocity through the restriction R, across the endsof which there is a pressure difference AP=P_(i)−P_(u) is given byF=k(P _(i) ² −P _(u) ²)/P _(u)in which k=k′D⁴/ηLwhere k′ is a proportionality constant, D is the tube diameter, L itslength and η the gas viscosity.

The flow regulator of the invention uses at the ends of the restrictionR two regulators 2 and 4 for regulating the pressures P_(i) and P_(u) atits respective ends.

More specifically, with reference to the flow direction F, the upstreamregulator 2 is a direct regulator, whereas the downstream regulator 4 isa back-pressure regulator.

The direct regulator 2 is in the form of an electronically controlledproportional valve 6 able to reduce its port size when the pressureP_(i) at the control point, which is positioned downstream of the valve,exceeds the set value. In this manner the valve 6 automatically returnsthe pressure P_(i) to the set value.

The back-pressure regulator 4 is an electronically controlledproportional valve 8 able to increase its port size when the pressureP_(u) at the control point, which is positioned upstream of the valve,exceeds the set value.

Again in this case the valve 8 automatically returns the pressure P_(u)to the set value.

Different ways of electronically controlling the proportional valves 6and 8 exist. One of these ways, advantageously usable in the device ofthe invention, is to use a program developed for processor Scorpion 128KController—5521 devised and marketed by Micro-Robotics Ltd, Cambridge.

By virtue of the aforedescribed circuit configuration, the pressurevalues P_(i) and P_(u) can be controlled independently, to hence, forequal temperatures, maintain a constant flow passing through therestriction R to determine the controlled pressure values at its ends.

With reference to FIG. 1, it is apparent that if the pressure P_(i)falls below the set value, the valve 6 opens to a greater extent to passan increased pressure and so restore equilibrium.

If instead the pressure P_(u) decreases, the valve 8 closes to reducethe discharge from the measurement point to the outside, and againrestore equilibrium.

The combined effect of the two independent adjustments enables the gasflow through the restriction R to be controlled by using two pressurecontrols and correct application of the Poiseuille equation (this infact assumes different exemplifications in relation to the linearvelocity of the gas).

It should be noted that the Poiseuille equation is in fact known fordetermining the value of a gas flow knowing the pressures across acircuit element traversed by said gas flow, however two pressureregulators of different type have never been used to regulate a gasflow.

Because of the facility for easily and at the same time accuratelycontrolling a gas flow, the device of the invention is suitable foradvantageous use in various chemical and industrial sectors, and inparticular in typical chromatographic applications, such as in astandard throttling circuit, an electronic control circuit for a gaschromatography column split, and a control circuit for column switchingin a multi-column analytical system.

Standard Throttling Circuit

In the chromatography field it is frequently required to obtainpredetermined concentrations of gaseous sample (standard) at differentconcentrations but at rigorously constant flow rates, to be then used inquantitative determinations.

For this purpose a gas cylinder 10 is used, for example of methane inknown standard concentration, for example 6 ppm, in helium.

From the scheme of FIG. 2 it can be seen that the gas from the cylinder10 passes through the device of the invention, indicated overall by 12and comprising a restriction R₁ interposed between a direct-pressureregulator 2 upstream and a back-pressure regulator 4 downstream.

The characteristics of the restriction R₁ and the set values of the twopressures P_(i1) and P_(u1) are chosen such that the gas flow F₁,through the regulator device 12 is for example 200 ml/min.

The standard throttling circuit also comprises another cylinder 14containing the actual gas in which the methane is dissolved, i.e. purehelium. From this cylinder there extend n separate branches passingthrough a further n flow regulator devices 12 of the invention, with nrestriction R₂, R₃, . . . , R_(n+1).

Let F₂, F₃, . . . , F_(n+1) be the gas flow rates through these ndevices 12.

For descriptive simplicity it will be assumed that the regulator devices12 are equal and that hence the flow rates F₁, F₂, F₃, . . . , F_(n+1)are all equal to 200 ml/min, although this limitation is in no wayessential.

If the flow rate leaving the throttling circuit is indicated by F_(u) itis immediately apparent that if the n branches are closed, F_(u)=F₁=200ml/min and the concentration of methane in helium is 6 ppm.

If one branch, for example that comprising the restriction R₂, is openand the other branches are closed, F_(u)=F₁+F₂=400 ml/min and theconcentration is 3 ppm.

If two branches, for example those comprising the restrictions R₂ andR₃, are open and the remainder are closed, F_(u)=F₁+F₂+F₃=600 ml/min andthe methane concentration is 2 ppm. and so on.

Each branch can be closed by acting for example on the relativeback-pressure regulator 4.

As there are also other branches, it is evident that by varying thenumber of these and by varying the characteristics determinable by eachflow regulator, and starting from a single gas cylinder 10 of standardconcentration, practically any lower standard concentration value can beobtained without having to vary the total flow rate.

Spit Control Circuit

FIG. 3 schematically illustrates an analytical circuit with a column 20to be inserted into an oven for chromatographic analyses.

The circuit comprises a cylinder 22 of transport gas, a flow regulatordevice 12 according to the invention, an injector 24 into which the gasto be analysed is fed, a purge circuit 26 and a split circuit 28, theseextending from the injector 24, and an analytical circuit comprising theanalysis column 20 and an exit detector 30.

Both the purge circuit 26 and the split circuit 28 comprise arestriction R_(p) and R_(s) and a back-pressure regulator 4 positioneddownstream of said restriction R_(p) and R_(s).

As a direct-pressure control P_(inj) is provided within the injector 24,this forms with the two circuits 26 and 28 two regulator devicesaccording to the invention.

In operation, the inlet regulator device controls the flow F_(T),whereas the other two regulator devices 24-26 and 24-28 control thepurge flow F_(p) and the split flow F_(S) respectively, and in thismanner they also indirectly control the column flow F_(c), which is notdirectly controllable because the components to be analyzed pass intothe column in very small concentration, and it is easy to imaginecontamination of these samples as they pass through regulator valvescomprising gaskets.

Multi-Column Analytical Circuit

In gas chromatography it is often necessary to use analytical circuitscomprising several columns, which have to be able to be excluded inthose cases in which their presence could alter the use of others. Suchcircuits must therefore be provided with shut-off valves able toselectively exclude one or more columns, and they must at the same timebe provided with means able to prevent, on excluding one or morecolumns, any modification in the gas flow rate through the non-excludedcolumns.

It follows that if column switching is effected in an analytical circuiton the basis of analytical requirements, this result in a circuitmodification and a variation in the gas flow through the non-excludedcolumns. To preserve the original regime the pressure in the variousparts of the circuit must be varied, with the result that the originalconditions are restored with a certain transient delay, which inaddition to slowing down the measurement can also result in itsalteration because of possible loss of significant data during thetransient phase.

Moreover in chromatographic analyses, in which sensors are used whichare sensitive to variation in the gas flow to which they are exposed(for example TCD sensors), it is important that during the analysisthere is no flow variation which could alter the measurements.

Consequently in multi-column analytical circuits, there is interest bothin maintaining the transport gas pressure constant during columnswitching to prevent, or at least reduce, the said transients, and inmaintaining the transport gas flow constant in order not to alter thesensor response.

Up to the present time this has been achieved either by using calibratedhydraulic restrictions, which hence have an excessive rigidity as theircharacteristics can in no way be modified, or by using needle valvesadjustable manually by the operator, who must therefore be a person ofadequate technical knowledge and reliable experience.

The multi-column analytical circuit illustrated in FIG. 4 effectivelyovercomes these limitations by the use of a flow regulator deviceaccording to the invention.

This circuit uses as the restriction R of the regulator device theactual multi-column analytical circuit C₁ . . . . C_(n) itself, which istherefore interposed between the direct-pressure regulator 2 (upstream)and the back-pressure regulator 4 (downstream).

For the purposes of circuit operation, the circuit detector 30 can bepositioned either upstream or downstream of the back-pressure regulator4, even though for practically reasons it is advisable that it bepositioned downstream to enable the two regulators 2 and 4 to remainoutside the oven into which the analytical circuit C₁ . . . C_(n) has tobe inserted.

This special use of the regulator device of the invention isparticularly advantageous, in that it causes the detector 30 to operateat constant flow and at the same time the columns C₁ . . . . C_(n) tooperate at constant pressure; consequently all errors due to flowvariations through the detector are eliminated, and at the same time,because of the elimination of transients in column switching, the timerequired for analytical analysis is substantially reduced enablingchromatograms to be obtained with considerable reduction in systemerrors.

1. A flow regulator device for an analytical circuit, comprising: afluid restriction, a direct-pressure regulator positioned immediatelyupstream of said restriction, and comprising a proportional valvecontrolled electronically on the basis of the pressure existingdownstream of said valve, with reference to the direction of the flowpassing through said fluid restriction a back pressure regulatorpositioned immediately downstream of said restriction and comprising aproportional valve controlled electronically on the basis of thepressure existing upstream of said valve, with reference to thedirection of the flow passing through said fluid restriction.
 2. Adevice as claimed in claim 1, wherein said fluid restriction comprises apiece of tube of constant diameter.
 3. A flow regulator device for ananalytical circuit, comprising: a fluid restriction, a direct-pressureregulator having a port and positioned immediately upstream of saidrestriction, the direct pressure regulator controlling the pressureentering the restriction, the direct-pressure regulator port becomingsmaller when the pressure exceeds a preset value, a back pressureregulator having a port and positioned immediately downstream of saidrestriction, the back pressure regulator controlling the pressureleaving the restriction, the back pressure regulator port becominglarger when the pressure exceeds a preset value.